FIGS. 1 through 3 of the accompanying drawings will be used in describing problems addressed by the invention. Referring to FIG. 1, a gas turbine engine 10 is typically started by first starting an auxiliary power unit (APU) 12. In the illustrated scheme the APU 12, acting through a gearbox 14, drives an air compressor 16 that supplies pressurized air along a conduit 18 leading to an air turbine starter 20. Upon receiving a command signal to initiate startup of the engine 10, an electronic control unit 22 energizes a coil (not shown) which actuates open a valve 24, thus permitting pressurized air to be delivered to the inlet of the starter 20. The output shaft 26 of the starter 20 is drivingly engaged with the engine 10 through the gearbox 14. The starter 20 by itself provides starting torque for the engine 10 until the latter reaches its light-off speed, after which both the starter and the engine provide acceleration torque until the engine reaches some percentage of its design speed. At that point, the starter 20 should have reached its cutout speed. Attainment of the starter cutout speed is communicated to the electronic control unit 22 as a frequency signal originating from a proximeter-type sensor placed in the starter 20. When the electronic control unit 22 receives the appropriate frequency signal, it de-energizes the forementioned coil, which closes the starter inlet valve 24 so that the starter 20 stops running.
Turning to FIG. 2, there is schematically illustrated a conventional air turbine starter 20. When the starter 20 is running, pressurized air (indicated by arrows 30) is being supplied through an air inlet (not shown) to the turbine wheel 32 and is expanding through and imparting energy to the latter, thus exerting a torquing force on the turbine shaft 34 before departing through an air outlet 36 as exhaust. The torque is transmitted through a set of gears 38,40,42 to a geared hub 44. From the hub 44, the torque is transmitted through a sprag-type clutch 46 to an inner shaft 48 which is mechanically coupled to the output shaft 26. The starter 20 stops running when torque is no longer transmitted from the turbine shaft 34 to the output shaft 26. However, in a typical application such as that illustrated in FIG. 1, the output shaft 26 and the inner shaft 48 continue to rotate at high speed, being driven by the engine 10 after the starter 20 discontinues running. The overrunning clutch 46, however, operates to prevent torque transmission from the inner shaft to the hub 44. The post-cutout lubrication needs of the clutch 46 exceed the total pre-cutout lubrication needs of the starter 20. Accordingly, the starter 20 contains a volume of lubricant 50 which meets the post-cutout needs of the clutch 46, but which exceeds the pre-cutout needs of the starter. During the pre-cutout mode when the starter 20 is running, the lubricant 50 is churned by internal, rotating components of the starter and is a source of parasitic loss of torque. The degree of parasitic loss increases with the volume of lubricant 50 contained in the starter 20. If this loss can be eliminated or reduced, a number of advantages may be provided. To the end of explaining these advantages, attention is now redirected to FIGS. 1 and 3.
FIG. 3 is a generalized graph of output torque versus speed for an air turbine starter 20 employed as indicated in FIG. 1. A range of cutout speeds is indicated by opposing arrows 52. Assuming the compressor 16, the conduit 18, and the starter 20 are properly designed, built and installed so that the starter meets the starting torque requirements for the engine 10, the solid-line curve 54 represents the performance of the starter. However, over time, internal components of the compressor 16 are abraded. This may result in a significant decrease in pressure for the air supplied to the starter 20, and is of particular concern in applications such as military helicopters wherein the compressor 16 may be routinely exposed to abrasive airborne particulates. The decrease in available inlet pressure adversely affects the performance of the starter, as represented in part by the dashed line 56. If the decrease in pressure is sufficient to prevent the starter from reaching cutout speed, then the electronic control unit 22 will not receive a sufficiently high frequency signal from the sensor 28 to respond by closing the inlet valve 24. Consequently, the starter 20 continues to run until functionally destroyed by over-heating. Even if degradation in the performance of the compressor 16 is insufficient to prevent the starter 20 from attaining cutout speed, the performance of the starter may be sufficiently impaired to significantly increase the time required for the engine 10 to attain its normal operating speed.
Thus, to the degree that the performance of an air turbine starter is improved by reducing the forementioned parasitic losses, the following advantages or potential advantages are realized: first, a decrease in the number of starter failures for particular applications thereof; second, an increase in the time over which the compressor can be operated before replacement of abradable components is necessary in said applications; and third, an improvement in the overall performance of such air turbine starters, generally.